Science Buddies: "Ask an Expert"

Hello,
I have to make a rocket project for STEM. I was wondering what kind of rocket fins are the best to eliminate drag and causes the rocket to travel in a vertical straight line.
Any answers are greatly appreciated.

Hi cdoan,

Your question would actually make a great science project. How do fins change flight, and which fins are the best?

You could get a quick answer from making paper airplanes or rockets (as in this project: https://www.sciencebuddies.org/blog/pap ... rodynamics) and experimenting with fin sizes and shapes.

It's worth noting that fins usually increase drag. However, as you've noted, they can improve stability and help your rocket travel in a straight line, leading to higher launch heights.

The answer can be complicated. The best fins will depend on material, speed, and size of your rocket, as well as other factors. If you define your operating conditions, you'll be able to narrow down your experiments.

Best,
Charles

cdoan,

Model rocketry can indeed be a good science project. As charlesg points out, fins are actually designed to increase drag. This may seem like a bad thing, but they help ensure stable flight. The question is, how big or how little should the fins be to minimize drag, while providing enough stability to optimize flight trajectory?

When exploring this aspect of rocket design, there are two key principles to keep in mind. Center of Gravity (CoG) and Center of Pressure (CoP).

CoG is the point in the rocket (or any object) where exactly half the mass of the object is on one side and half on the other. This is the point that the force of Thrust, pushing the rocket forward, acts through.
CoP is the point on the rocket where half the surface area of the rocket is on one side and half on the other side. This is the point that the force of drag, pushing back against thrust, acts through.

Ideally, with a rocket, you want the CoG closer to the nose and CoP closer to the tail. The further apart these point are the more stable your flight will be. If they are two close together or worse, CoP is closer to the top than CoG then the rocket will be very unstable and likely to end up in a tumble as the thrust tries to push the CoG "in front of" CoP and drag tries to pull CoP back behind CoG during flight.

CoG can be changed by adding/subtracting mass from the payload inside the nose cone. CoP can be changed by adding/subtracting surface area to increase/decrease drag. This can be done by adding/subtracting fins and/or changing the size and shape of them.

Note: Symmetric design is important (i.e. don't put one giant fin on one side opposite a small fin). Your CoG and CoP may be set correctly, but the asymmetric design will still cause unstable flight).